The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for improved position error signal (PES) demodulation in the presence of baseline wander offset in disk drives.
Modern disk drives are becoming more sophisticated with lower flyheight, higher areal recording densities, and faster rotational spindle velocities. Many new high capacity hard disk drives (HDDs) are storing an increasingly large amount of data on the magnetically coated disk surfaces. The areal recording densities are exceeding 10 Megabits per square inch. This large areal recording density requires the recording heads to fly even closer to the disk surface. The reason for the lower flyheight is to obtain the necessary signal strength in the readback signal from the transducer MR-head to discern the recorded data-signal from noise. In other words, the signal-to-noise (S/N) ratio in the readback signal has to exceed a certain level for reliable reading and writing of the recorded signal.
Average flyheights of less than one microinch or 25 nanometers are common. The dynamic flyheight separation between the head transducer and the disk surface can be considered a random process with a mean value equal to the average flyheight and an erratic variance. Factors involved in the flyheight variation are topological disk surface variations, turbulence in the airflow making up the airbearing cushion underneath the slider, mechanical resonances in the actuator arm/suspension structure, and the like.
A functional MR head is biased by a current passing through the MR element. The MR bias current that is roughly 10 milliamperes is heating the MR head. The overall readback signal from a MR head consists of two components, the magnetic and the thermal. Research, by the present inventors, has identified any changes in the baseline of the readback signal from a MR head, as being caused by the thermal signal component. The low frequency thermal signal component is often called the baseline-wander. Any variations in the thermal signal component are caused by minute variations in the heat transfer between the MR head and the disk surface. The heat transfer varies with the physical separation between the heated MR head and the cooler disk surface. MR bias current that is roughly 10 milliamperes is heating the MR head. When the physical separation is reduced, as would be the case of a protruding defect passing underneath the MR head, the MR head will cool down slightly. If a surface pit slides by the MR head, the separation is increased and the MR head will heat up slightly. Due to the resistive temperature coefficient of the MR head, these minute cool-downs and warm-ups will change the voltage across the MR head. This change is the thermal baseline-wander.
Today most hard disk drives (HDD) have prerecorded sectors of servo information, written magnetically, on each of the disk surfaces. Typically the servo information is demodulated into a position error signal (PES) and used for radial positioning of the recording head. This information, for example, is written in 50-100 equally spaced sectors that are slightly off perpendicular to the concentric circular tracks. Around the track, the servo-sectors are interlaced with much larger data-sectors.
The occurrence of a thermal asperity (TA) in the servo sectors can result in large transient offsets in the readback signal, that is, the baseline wander changes rapidly. As used in the following specification and claims, the term thermal asperity (TA) should be understood to include media dropouts and other grown defects. Media dropouts and other grown defects in the servo sectors can result in incorrect position error signal (PES) demodulation, causing unacceptable track misregistration (TMR) problems in high performance disk drives.
A need exists for a method and apparatus for improved position error signal (PES) demodulation in the presence of thermal asperities.
A principal object of the present invention is to provide a method and apparatus for improved position error signal (PES) demodulation in the presence of thermal asperities. Other important objects of the present invention are to provide such method and apparatus for improved position error signal (PES) demodulation in the presence of thermal asperities substantially without negative effect and that overcome many of the disadvantages of prior art arrangements.
In brief, a method and apparatus are provided for improved position error signal (PES) demodulation in the presence of thermal asperities. A plurality of samples is obtained within each period of a burst signal for each servo burst in a servo sequence. A difference value is identified between a largest sample value and a smallest sample value of the plurality of samples within each period of the burst signal. The difference values for each period of the burst signal for each servo burst in the servo sequence are summed to produce a burst sum value for each servo burst. Then the burst sum values are used to identify a position error signal (PES) value.
In accordance with features of the invention, the identified PES value is independent of any transient baseline voltage offset disturbances, for example, resulting from the occurrence of a thermal asperity (TA) in the servo sectors.